A rat race coupler (also known as a hybrid ring coupler) is a type of coupler used in RF and Microwave systems. In its simplest form, a rat race coupler is a 3 dB coupler and is thus an alternative to a magic tee. Compared to the magic tee, it has the advantage of being easy to realize in planar technologies such as microstrip and stripline, although waveguide rat races are also practical.
The rat-race coupler has four ports, each placed one quarter wavelength away from each other around the top half of the ring. The bottom half of the ring is three quarter wavelengths in length. The ring has a characteristic impedance of factor √{square root over (2)} compared to port impedance, for example.
Rat-race couplers are used to sum two in-phase combined signals to equally split an input signal with no resultant phase difference between its outputs. Rat race couplers usually do not have such a wide bandwidth and are constrained by their designed frequencies. As to this latter point, a rat race coupler has a single frequency, which can be used with a single device. To accommodate different frequency devices, it is necessary to use a different rat race couplers. Accordingly, large chip area needs to be used to accommodate different operating frequencies.